1. Field of the Invention
This invention relates to a pneumatic fastener driving tool, and more particularly to an improved piston for use in such a tool.
2. Background of the Invention
Pneumatic tools for driving fasteners such as nails, staples, brads and the like are commonly used in the commercial work place. All of these devices have standard components comprising a housing to store compressed air, a cylinder in which a piston and driver combination are driven in a reciprocating manner, a valve means to provide pressurized air to the piston and a fastener carrier means to position the fastener underneath the driver prior to the driving stroke.
Most tools are operated by positioning the tool in contact with the workpiece and manually pulling a trigger which in turn operates a valve means that provides compressed air to the top side of the piston. When the tool is used as a stationary device the trigger is replaced by a remote actuator.
As the piston reaches the end of the driving stroke, a return air chamber is pressurized to provide air for the return stroke of the piston and driver. After the trigger is released, the valve closes blocking air into the top of the cylinder and in turn opens an exhaust port to release the air above the piston to the atmosphere. The stored air within the return chamber acts upon the underside of the piston to return in to the rest position at the uppermost portion of the cylinder.
To provide enough power to drive the fastener, the air must enter the cylinder above the piston quickly. To accomplish this, the valve means is normally divided into two functions. A firing valve is located directly above the top of the cylinder and is shifted from a closed position pneumatically by a trigger valve. By utilizing air pressure, the firing valve can be held closed tightly and then opened with a snap action when air pressure on a portion of the valve is reduced.
Although the firing valve is designed to open quickly, the air starts to enter the area above the piston as soon as the valve begins its initial movement. This causes the piston to begin its movement at a pressure much less than that within the compressed air chamber of the housing due to air flow restriction between the cylinder and the firing valve. The piston does not achieve the desired velocity until a portion of the total drive stroke is used, and thus, does not develop maximum power.
In order to delay the start of the piston until the valve opens further and the pressure on the top of the piston builds up, it is desirable to hold the piston momentarily in its uppermost position. This feature has been achieved as disclosed in U.S. Pat. No. 3,397,617. Although the described means accomplishes the delay action, it introduces an undesirable condition in that air is lost between the outside of the piston and the inner cylinder wall.
Many applications using portable fastener driving tools have a limited air source. Any air not used during the driving cycle is costly and restricts the amount of fasteners that can be driven before the air source can be replenish with the air that is lost. The loss of air between the piston and the cylinder can be eliminated by the addition of a seal mounted on the piston and having an air sealing surface in contact with the inner wall of the cylinder.
The most commonly used seal is an O-ring, which would stop the loss of air by placing the O-ring in frictional contact with the inner wall of the cylinder. However, the O-ring sliding against the cylinder wall could cause an even more serious problem. Specifically, the air sources normally used for these type of pneumatically operated tools are subject to dirt and other contaminates that shorten the life of such seals. Also, the lack of proper lubrication causes excessive wear on seals and usually results in the tool losing power with use, or failing to function altogether. Frictional movement of pneumatic seals is the cause for most service calls for these tools.